1. Field of the Invention
This invention relates to nuclear reactor containment arrangements and more particularly to permanent seal rings for establishing a permanent seal across the thermal expansion gap defined between a peripheral wall of a nuclear reactor vessel and a containment wall.
2. Discussion of the Art
The refueling of pressurized water reactors is an established operation which is normally carried out with a high degree of reliability. In such reactors, a reactor vessel is positioned in a cavity formed by a concrete containment wall. The containment wall has a lower annular portion or well in which the vessel is seated and an upper annular portion above the vessel which defines a refueling canal. The canal is maintained dry during reactor operations, but is filled with water during the replacement of spent fuel assemblies. During the refueling, the reactor vessel is opened, thereby exposing the core, and fuel assemblies are exchanged. The water in the refueling canal acts as shielding to maintain radiation levels within acceptable limits.
The reactor vessel has flanges which join a head assembly to the reactor vessel proper. A lower flange is positioned on the peripheral wall of the reactor vessel near the vessel opening and is sealed to the lower portion of the refueling canal by a seal ring. The seal ring prevents leakage, during the refueling operation, of shielding water into the well in which the vessel is seated. The seal ring is usually preferably water-tight, however, pumps are provided in the well for expelling unwanted fluids, such as water which might penetrate the seal if the sealing engagement thereof is imperfect. However, shielding water leakage has more serious consequences than potential damage to the well or vessel. A sudden loss of shielding water from the refueling canal exposes personnel working in the area to high radiation dosages and contaminates the environment. Such a sudden loss of shielding water does not give personnel sufficient time to make safe the exposed radiation sources and must be prevented.
Prior art seal rings are discussed in U.S. Pat. No. 4,170,517 to Meuschke et al, the disclosure of which is herein incorporated by reference. Removable gasket-type seal rings, such as elastomeric gaskets, are known and are installed after reactor cooldown and prior to the flooding of the refueling canal. Such elastomeric gaskets are susceptible to thermal degradation and leakage, require significant amounts of time to install, and must be replaced at each refueling operation.
Permanent seal rings are also known and have the advantage of reducing the time required for the refueling operation. The Meuschke et al patent discloses a permanent, water-tight, flexible seal having a quarter-circular cross-section, made of, for example, stainless steel, and being constructed to cyclicly contract and expand, respectively, with cyclic expansion and contraction of the reactor vessel relative to the containment wall during reactor operation, while maintaining the water-tight integrity of the seal. Such a seal is inherently flexible across its entire expanse, however, and the strength of the member is limited by this flexibility requirement. Thus, such seals are susceptible to damage from falling objects, such as accidentally dropped fuel assemblies. Such accidents can involve a sudden and nearly complete loss of shielding water from the refueling canal, thereby endangering personnel and contaminating the environment.
Accordingly, a new permanent seal ring is needed which is strong and flexible. The new seal ring must be structurally strong enough to withstand the impact of the heaviest object which might inadvertently be dropped thereon, without a complete or sudden loss of shielding water ensuing, and preferably with the sealing integrity being substantially maintained. The new seal ring must also be structurally flexible enough to accommodate the radial and axial thermal expansion/contraction of the reactor vessel relative to the containment wall experienced during reactor operation, while substantially maintaining its sealing integrity, so that the installation may be viably permanent.